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Is there a safe way to power my house using a portable generator? The power rating for the generator is 6kW, and I want to run my electric water heater (4.5kW) and my refrigerator (0.5kW) and possibly a few lights.

The water heater is the main priority, but it's wired directly to the house so I can't just unplug an outlet and plug it into the generator.

The method I've heard used most often is to backfeed into the dryer outlet after turning off the main breaker. I'm aware that if I don't turn off the main breaker, I could shock a worker or destroy the generator when the power comes pack on. I also know enough that I have to use a power line rated for the Amperage I'll be drawing. Are there any other dangers I need to look out for?

I had a friend suggest that I deactivate one of the heating elements and thereby reduce the amount of power that the water heater will draw. I'm guessing I'd still need 240V rather than 120, but I'm not sure. It would be awesome if I could just use a 12 or 14 gauge extension cord instead of a really expensive 10 gauge, but I don't want to do anything that will catch my house on fire.

Also, can anyone give me a detailed explanation of steps I need to take for this whole process?

(Any advice would be greatly appreciated. It's been eight days since we've been without power, and possibly 3-4 more until it's restored.)

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Here's a really good list of the dangers of doing this: qsl.net/kc5qhh/backfeeddangers.pdf –  gregmac May 5 '11 at 21:33
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WHERE ARE THE DIRE WARNINGS in the answers here against doing this? NEC requires you use a transfer kit/interlock to abolutely ensure you can't feed power back onto the grid (amplified to 17,000 volts or so by going backwards through your transformer, by the way). This seems to be a much better answer. –  Craig Nov 28 at 20:47

5 Answers 5

up vote 13 down vote accepted

You've got the big one covered, turning off the main breaker. Without that, the voltage from the generator goes back out the power lines, into the transformer, where instead of getting stepped down to your house voltage will get stepped up to the line voltage, plus make previously dead wires, live.

Given the options, using a dryer style plug for 220v is the way I would do it in your situation. Back feeding 110v power will only send current down half of the breakers, and it would be easy to swap the neutral and the hot in this situation (with 220v you have two hots, and ground and neutral are effectively the same wire). That said, the proper way is with a transfer switch that would prevent power from back feeding into the power mains and also a plug that doesn't allow for live exposed prongs. These can be installed, but the time to do so isn't when you've been out of power for days. Just realize that using the dryer plug is almost certainly out of code compliance in your state.

Words of caution include checking the amperage of your wiring (the breaker on the dryer will have the number going to the plug) and then avoid overloading the generator. I wouldn't open up more breakers to major appliances than you have amperage to cover (e.g. if the breaker to the hot water say 20 and the dryer breaker says the same, then run that device by itself unless you have a good understanding of how many amps it actually pulls). You can use the breakers to power one device at a time, fridge for a few hours, then the hot water, etc. And when the water is hot and fridge is cold, shutoff the generator to save gas and leave the fridge closed to keep it cool.

Only other piece of advice I can think of is to make sure the exhaust is properly vented. Far too many people don't think and run these indoors, giving themselves carbon monoxide poisoning.

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Don't forget that refrigerators can pull a lot of power when they're starting, IIRC something like 2 kW instantaneous. –  Niall C. May 5 '11 at 20:06
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Transfer switch is the way to go. –  Tester101 May 5 '11 at 21:00
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@Tester101: Absolutely, but when you're dealing with a natural disaster, getting a transfer switch installed may take longer than desired. –  BMitch May 5 '11 at 21:53
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@B Mitch: true enough. In that case I would say do what you have to to get through, then have a transfer switch installed so your ready the next time. But watch out for the angry mobs and looters when your the only house on the block with power, make sure you lock down that generator so it doesn't walk away. –  Tester101 May 6 '11 at 12:09
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I actually decided against backfeeding since it's a borrowed generator, and we have friends with power 10 minutes away who have graciously allowed us to shower at their house. Good answer though, and thanks! –  Doresoom May 6 '11 at 13:41

You know what, I'm going to repeat the answer found here to the same question. Here's that answer, short and sweet and to the point:

ABSOLUTELY NOT!! This is NEVER an option.

You MUST use some form of transfer switch or
interlock, along with the proper male inlet.
Also, a male-to-male cord is called a
"suicide cord" for a reason.

A transfer switch/interlock wired into your service panel is the ONLY LEGAL WAY to do this, and the only safe way.

Look, transfer switches.

Just a few expanded reasons not to do this include:

  • Check out this post, where a retired lineman talks about how many times he's seen broken main breakers that still let current through even when they were OFF. Yikes.

  • Backfeeding your generator into your dryer outlet is ILLEGAL, possibly criminally so, or at least totally open to civil prosecution. Tell me that an injury caused by you turning on your generator and feeding power back onto a damaged electrical grid where people are working to repair the lines, and into your neighbor's homes during a crisis won't be actionable? The NEC requires a transfer switch (and not for trivial or capricious reasons). That's good enough for me.

  • the "suicide cord":

    • Seriously, do you actually WANT a #10 extension cord energized with 240V/30A strung across your floor with energized metal prongs sticking out the end of it? What if it accidentally gets kicked out of the wall socket or something? A million things could go wrong. HORRIBLE idea.
  • You'll dump power (stepped up to 17,000V by the transformer outside, so it'll go for miles and miles) on the power grid and electrocute a lineman doing repairs.

    • You, or more likely a spouse, visiting relative or one of your kids, will forget to throw the main breaker before powering on the generator. Or one of your kids' rambunctious friends will fire up your generator when you're not looking, on some sunny July afternoon.

    • Guaranteed, especially since the generator will be needed seldom enough that nobody will remember the entire power-up checklist when the time comes.

    • The whole point of the transfer switch being an interlock is that it only physically permits a connection to one power source at a time.

  • You'll fry your generator and/or anything with electronics in your home:

    • When you've inevitably forgotten to throw the main breaker before powering up the generator, and you've been lucky enough not to have electrocuted a lineman, when the main power comes back on there's a good possibility it will backfeed into your equipment and cook it in one way or another (the timing of the 60Hz wave will be different, possibly amplifying the power to much higher levels, etc., other interesting possibilities).

If you do this, and none of the bad things happen to you, that's what we call blind dumb luck and you really shouldn't tempt fate like that more than once. :-)

Don't do this. It's illegal and just a terrible idea--"Redneck" in the worst way.

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His problem happened back in 2011. Hopefully he's got normal power restored by now, and maybe even got a proper transfer switch installed. Chances of finding a transfer switch in the middle of a regional, long-term blackout are more-or-less zero, so saying "don't" isn't always an option. –  paul Nov 29 at 4:47
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His problem occurred in 2011, but the next person looking for sound advice here might be having their crisis tonight, or tomorrow. I don't really agree that "don't" isn't always an option. You can absolutely run extension cords from the generator to the furnace and fridge, maybe the range, maybe the Internet router or chargers for your smart phones, and that'll get you through, then you know to do it correctly when things clear up so that your life can be convenient during the next crisis or simple outage. –  Craig Nov 29 at 5:33
    
Honestly, it has got to be easier to pull together a few regular extension cords than to get a #10 suicide cord put together with the correct 240V male ends to backfeed through your dryer outlet. –  Craig Nov 29 at 5:35
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@paul - Daying "Don't" is always an option. If he'd asked if he could temporarily light a fire in the middle of his basement to heat the house, the only answer would be "don't do that because the smoke and carbon monoxide will kill him and his family". When something is so dangerous that it's outright illegal in many localities, besides being a clear code violation, the right answer is "don't do it". The worst time to try out some risky procedure is during a disaster since rescue services may be delayed. If you want to power your house from a generator, do it the right way. –  Johnny Nov 29 at 20:06

A transfer switch is the only way to go, it's only a few hundred bucks, the fee for an attorney for one hour. If you kill someone while back feeding your generator, the cost of a transfer switch is a drop in a bucket.

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I can't believe your answer has gotten zero votes. Using a transfer switch/interlock is the only LEGAL way to do this. –  Craig Nov 28 at 20:48

On your water heater that uses 220V and has two elements, you will find that only one element is ever on at a time. When the top thermostat is satisfied, it will send power down to the lower thermostat. If you ever have a problem with the top thermostat or element you will have no hot water. If the lower thermostat or element has a problem you will find you don't have as much hot water as you should. So you are not saving power draw by disconnecting an element.

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Good to know, but my main question was backfeeding a generator, which I ultimately decided against. –  Doresoom Mar 9 '13 at 18:20

What you are suggesting with a back-feed cord is possible, but not recommend for a bunch of reasons, including the ones you mentioned.

The first thing that comes to mind in terms of simplicity is replacing the power wire on the water heater with a cord made for a dryer and plug it in to the generator. When your power crisis is over, you can either wire the hot water heater back, or install an outlet.

The next best thing to do i to buy a transfer switch sub-panel, install it, and move the circuits you want to keep using to that sub-panel.

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